1. Field of the Invention
The present invention relates to an apparatus for machining holes in a workpiece by using plural condensed laser beams.
2. Description of the Background
A conventional apparatus employs condensed pulsed laser beams to make very small air circulation holes in filter wrapping paper. Such apparatus is disclosed in Japanese Patent Disclosure Publication No. 42200 of 1980, Japanese Patent Disclosure Publication No. 9784 of 1983 and Japanese Patent Disclosure Publication No. 180290 of 1991. Also well known is an apparatus which uses condensed pulsed laser beams to make very small holes in order to make the packing materials of fabrics or food, seasonings, etc., breathable and ensure ease of unpacking.
Such apparatuses known in the art generally make a string of holes in a band-shaped material by condensing and applying pulsed laser beams to the material as the material moves at high speed.
FIG. 12 is a diagram showing the arrangement of a laser cutting apparatus known in the art. FIG. 13 illustrates the changes of pulsed laser beam output in relation to time, and FIGS. 14a and 14b comprise a laser beam condensing status diagram, showing top and side views.
Referring to these drawings, the numeral 1 indicates laser oscillator, 2 denotes a pulsed laser beam, 3 represents a beam splitter, 4 indicates bend mirrors, 5 designates condenser lenses, 6 is a band-shaped material, 7 denotes transfer rollers, 8 represents beam-condensed spots, and 9 shows holes made in the material 6. In FIG. 14b, 10 indicates a space energy distribution of the pulsed laser beam. FIG. 14b shows energy distribution in a single mode, i.e., Gaussian distribution having a single peak.
The operation of the conventional apparatus will now be described with reference to these-drawings. The output of the pulsed laser beam 2 from the laser oscillator 1 is an intermittent, approximately rectangular wave as shown in FIG. 13. There are a wide variety of pulsing methods that are well known in the art, e.g., a method wherein the exciting action itself of laser oscillation is pulsed and a method wherein continuous oscillation is mechanically chopped to create pulses, and therefore these methods will not be detailed here. The pulsed laser beam 2 is divided by the beam splitter 3 and directed along separate paths. In a case as shown in FIG. 12, beam 2 is divided into four quarters and the four pulsed outputs are deflected by the corresponding bend mirrors 4 and fall on the corresponding lenses 5. The pulsed laser beams impinging on the lenses 5 are condensed and form beam-condensed spots 8 on the bend-shaped material 6. Meanwhile, the bend-shaped material 6 or a workpiece is transferred by the rollers 7 in the direction of the arrow and the strings of holes are made in the band-shaped material 6 at intervals determined by the laser beam pulse frequency and the moving speed of the band-shaped material 6.
Where higher productivity is desired, the moving speed can be increased but the pulsed light must operate at a higher frequency if closely spaced holes are desired. One suggested approach is to divide the beam after it passes through the condenser lens 5, by using a beam-splitting prism lens (not shown). FIG. 15 illustrates output contour lines that are generated when such a beam-splitting prism lens is employed under the lens 5 in FIG. 12 to split the pulsed laser beam having single-mode energy distribution in the laser cutting apparatus shown in FIG. 12. As illustrated, when the single-mode laser beam is split by the prism lens, fringes 13a, 13b are produced on the workpiece by diffracted light, in addition to the two beam-condensed spots indicated by 8a, 8b. Since the fringes do not contribute to hole making, output loss occurs and hole making speed reduces substantially.
The laser cutting apparatus, which is arranged as described above, may increase the transfer speed of the band-shaped material to enhance production capability. However, such a conventional apparatus may require an increase in the output of the pulse laser beam to shorten the rise and fall times of the output. Consequently, a large laser oscillator is required to increase the laser output, resulting in high price. In addition, there are limitations based on the principles of laser oscillation and on the mechanical structure of the mechanical chopper which produces the pulsed beam that limit the output rise and fall times and the pulse frequency, preventing the improvement of productivity.